Larch Budmoth Outbreaks in the European Alps

Larvae of the larch budmoth (Zeiraphera diniana Gn.) -- or LBM -- feed on the foliage of sub-alpine larch (Larix decidua Mill.) trees that grow in a huge swath of forest across the vast expanse of the European Alps, where large outbreaks of the voracious insects have historically occurred every eight to nine years (Baltensweiler et al., 1977). These cyclical oscillations, as Buntgen et al. (2009) describe them, "are notorious, both in terms of regularity as well as the copious amounts of data collected on these oscillations during the middle part of the 20th century (Baltensweiler and Rubli, 1999)."

To determine in what ways and to what degree this cyclical phenomenon might possibly be related to changes in climate, Buntgen et al. analyzed raw measurements of 3151 tree-ring width (TRW) series and 150 maximum latewood density (MXD) series from 70 larch host sites and 73 spruce non-host sites spread across the European Alps and Tatra Mountains that extended back in time approximately 300 years.

Focusing on the European Alps -- "because periodic growth depressions caused by LBM outbreaks were not found in the TRW and MXD chronologies from the Tatra Mountains" -- the authors discovered "a distinct periodicity at 8-9 years from sites at elevations between 1750 and 1900 meters above sea level." In the middle of the 20th century, for example, they documented synchronized outbreak pulses at approximately 1936, 1945, 1954, 1963, 1972 and 1981. These outbreaks, in their words, "occurred independently of rising temperatures," all the way from the beginning-of-the-end of "the Little Ice Age until recent warmth." However, they report that this incredibly long stretch of persistent and synchronized Alpine-wide defoliation -- which Esper et al. (2007) had followed all the way back to AD 832 -- finally "ceased during recent decades," when "unprecedented warming in all seasons characterized the post-1980 period."

Although it is probably too early to attribute the cessation of synchronized LBM-induced defoliation of larch trees throughout the European Alps to the unprecedented regional warmth of the past quarter-century, the uniqueness of the break in the long record of LBM destructiveness gives one pause to wonder if such might not be the case. Another possibility arises from the fact that atmospheric CO₂ concentrations of the past quarter-century have been truly unprecedented, and on a global basis, for much more than just the past 1200 years. Consequently, the suppression of LBM outbreaks throughout the European Alps over the past quarter-century may well have been the result of some synergistic consequence of the two factors (high regional temperatures and global CO₂ concentrations) acting in unison, while a third possibility may involve only the increase in the air's CO₂ content.

Whatever the answer, one can be confident of one thing: if larch budmoth destructiveness throughout the European Alps had dramatically increased over the past quarter-century, CO₂-induced global warming would have been blamed for the problem in a heartbeat, and without any equivocation whatsoever.

Additional References
Baltensweiler, W., Benz, G., Bovey, P. and Delucchi, V. 1977. Dynamics of larch bud moth populations. Annual Reviews on Entomology 22: 79-100.

Baltensweiler, W. and Rubli, D. 1999. Dispersal -- an important driving force of the cyclic population dynamics of the larch bud moth. Forest Snow and Landscape Research 74: 3-153.

Esper, J., Buntgen, U., Frank, D.C., Nievergelt, D. and Liebhold, A. 2007. 1200 years of regular outbreaks in alpine insects. Proceedings of the Royal Society B 274: 671-679.